The invention pertains to the field of surface processing of materials and in particular to the use of plasmas in surface processing of materials
The matter of preparing a surface for further processing is an integral step in many industrial processes, and a vast array of methods and techniques exist to address this matter. At one extreme of the spectrum there are sophisticated techniques involving advanced ultra-high vacuum and ion-beam equipment to obtain surfaces that are near to atomically perfect, such as those required in the semiconductor industry. At the other extreme, there are macroscopic abrasive techniques such as sanding, which also have their specifically appropriate fields of application.
In the field of paper products a variety of methods have been employed to address the modification of surfaces by the removal of outer layers, for example, the removal of all or portions of coatings that may have been applied to a substrate. An example of an application in paper products is the bonding of packaging materials that have already been printed. In order to fold and glue the packaging, sections of printed area need to be removed and the surface prepared for good adhesion. In this kind of industrial field abrasion (for example, sanding), chemical treatment, and corona discharge treatment have all found application in one way or another.
Amongst the disadvantages of abrasion is the fact that it is a contact method, exercised by mechanical means. This leads to dust problems and considerable wear and tear on materials, parts and equipment. It is also difficult to control abrasive processes to a degree that allows extremely precise removal of outer surfaces, a feature that may be desirable in applications where it is important not to damage the underlying substrate, or where the application may require the removal in precise patterns or to specific depths. Abrasion is, however, a very direct and low cost method.
Chemical treatment, for its part, tends to be very selective in what it does or does not remove, and its efficacy will depend on the ability of the treatment to interact with the particular materials and surfaces involved. If the treatment involves the wet application of chemicals, there may be wetting problems associated with the process: for instance, when the particular treatment inadequately wets the materials to be removed, or else is absorbed by the underlying substrate, causing unwanted chemical changes or physical deformations (e.g., cockling in the case of paper products). Adsorption of chemical treatments may also leave unwanted residues. Chemical treatment also has associated chemical control and safety considerations, often governed by stringent regulations requiring special control mechanisms.
Corona treatment, while a very elegant physical technique, cannot remove materials to the degree required in many industrial applications and certainly is, for example, not capable of stripping sections of packaging materials prior to automated industrial glue bonding. The same holds for the wider spectrum of glow discharge techniques.
Various techniques based on light have been applied in this field and, while contact-less and highly directable, they tend to be expensive and quite selective about the materials that can be removed. Such techniques most often find application in the very highest technology arenas such as surface photo-preparation of semiconductors. In keeping with the specific requirements of these fields, they are then also often implemented in vacuum. This immediately limits the efficacy of these techniques within a broader base of industry. While high power light sources capable of operating in air at atmospheric pressures are available, they are very expensive.
In the case of surface treatments that can be used on a manufacturing scale, what is required is a non-selective, contact-less technique that does not require a special environment (e.g., a vacuum), and can be used on a wide variety of materials. The method must also be one that can work economically at very high speeds while still being directable in order to obtain maximal control over its application.
In accordance with the present invention, a directed plasma beam is employed in air to selectively remove coatings from paper products at high production rates. The shape and intensity of the beam is controlled to obtain a controlled rate of removal of the coating. The method does not require vacuum to be established and allows for the plasma to be generated from high pressure air.
FIG. 1 shows a directed plasma beam employed to selectively remove coatings on a paper-based surface moving at high speed.